1. Field of the Invention
The invention relates to a semiconductor light-emitting device and a method of manufacturing said semiconductor light-emitting device. The semiconductor light-emitting device can be utilized as, e.g., a light-emitting diode and a laser diode.
2. Description of the Related Art
Light-emitting devices using compound semiconductors are known as those covering visible to short wavelength regions. Among others, group III nitride semiconductors have attracted attention in recent years, not only because these semiconductors are of direct transition type, so that they exhibit high light-emitting-efficiency, but also because these semiconductors emit blue light, which is one of the three primary colors.
One example of such light-emitting device is formed by laminating an AlN buffer layer, a first clad layer, a light-emitting layer, and a second clad layer sequentially on a sapphire substrate. Here, the first and the second clad layers are made of AlXInYGa1xe2x88x92Xxe2x88x92YN (including X=0, X=Y, X=Y=0). The light-emitting layer has a superlattice structure formed by laminating a barrier layer made of InY1Ga1xe2x88x92Y1N (Y1xe2x89xa70) and a quantum well layer made of InY2Ga1xe2x88x92Y2N (Y2xe2x89xa7Y1 and Y2 greater than 0) repetitively.
These semiconductor layers are formed in accordance with an ordinary technique based on a metal organic vapor phase epitaxial growth method (hereinafter referred to as the xe2x80x9cMOVPE methodxe2x80x9d).
The thus superlattice-structured light-emitting layer, requiring steepness in difference of composition between the barrier layers and the quantum well layers, is formed at relatively low growth temperatures. Further, the respective barrier layers are generally formed to have the same thickness, and similarly the respective quantum well layers are formed to have the same thickness. This is because there is a danger that the wavelengths of light emitted from the respective quantum well layers will be slightly varied by the quantum effect if thicknesses differ between layers.
On the other hand, the second. clad layer that is formed on the light-emitting layer is formed at higher temperatures than the light-emitting layer in order to meet thickness and composition requirements (the second clad layer is thicker than the barrier layers and the quantum well layers).
The inventors have found that the following problems have been addressed in manufacturing the semiconductor light-emitting device.
In the superlattice-structured light-emitting layer, if layers adjacent to the respective clad layers are quantum well layers, the following problems are encountered. When a clad layer is of the p-type and a quantum well layer is contiguous to such clad layer, the depth of the well of such quantum well layer differs from those of the other quantum well layers because the clad layer has a different energy level from a barrier layer. Therefore, there is a danger that the wavelengths of light will be shifted. Further, if a clad layer is of the n-type and a quantum well layer is contiguous to such clad layer, the well is hard to form in such quantum well layer because the energy level of the clad layer is lower than that of the quantum well layer. As a result, emission of light cannot be expected.
To overcome the aforementioned problems, a first aspect of the invention is applied to a semiconductor light-emitting device that includes:
a first semiconductor layer that is made of n-type GaN;
a light-emitting layer of superlattice structure that is formed on the first semiconductor layer by laminating a barrier layer being made of InY1Ga1xe2x88x92Y1N (Y1xe2x89xa70) and a quantum well layer being made of InY2Ga1xe2x88x92Y2N (Y2 greater than Y1 and Y2 greater than 0); and
a second semiconductor layer that is made of p-type AlXGa1xe2x88x92XN (0.05 less than X less than 0.2), and
in such a semiconductor light-emitting device, layers that are adjacent to clad layers are the barrier layers in the light-emitting layer. That is, the light-emitting layer is designed to have a barrier layerxe2x80x94a quantum well layerxe2x80x94a quantum well layerxe2x80x94a barrier layer.
It may be noted that the first semiconductor layer and the second semiconductor layer correspond to clad layers or optical guide layers in the following description. It may further be noted that impurities introduced due to the background at the time of growing the semiconductor layers such as the barrier layers and the quantum well layers are not intentional impurities.
However, when the inventors have examined again, the following problems have further been found out.
When the second clad layer is formed on the light-emitting layer of superlattice structure, the barrier layer that comes on top of the light-emitting layer (hereinafter referred to as the xe2x80x9cuppermost barrier layerxe2x80x9d) becomes thin. The reason therefor is assumed to be as follows. Since the second clad layer is formed at higher temperatures than the uppermost barrier layer, the material of which the uppermost barrier layer is formed is blown away from the upper surface of the second clad layer at the time of forming the second clad layer.
It is not desirable that the uppermost barrier layer become thin, because the wavelengths of light are shifted toward the short wavelength side by the quantum effect.
Further, if the barrier layer is designed to be thin (e.g., to a thickness of some nanometers), there is a danger that no uppermost barrier layer substantially exists.
To overcome this problem, a second aspect of the invention is applied to a method of manufacturing a semiconductor light-emitting device that includes the steps of:
forming a first semiconductor layer that is made of AlXInYGa1xe2x88x92Xxe2x88x92YN (including X=0, Y=0, X=Y=0);
forming a light-emitting layer of superlattice structure that is formed on the first semiconductor layer by laminating a barrier layer being made of InY1Ga1xe2x88x92Y1N (Y1xe2x89xa70) and a quantum well layer being made of InY2Ga1xe2x88x92Y2N (Y2 greater than Y1 and Y2 greater than 0); and
forming a second semiconductor layer that is made of AlAInBGa1xe2x88x92Axe2x88x92BN (including A=0, B=0, A=B=0) on the light-emitting layer, wherein
an uppermost barrier layer, which is an uppermost layer of the light-emitting layer, is formed thicker than the other barrier layers.
Further, a third aspect of the invention is applied to a method of manufacturing a semiconductor light-emitting device according to the second aspect of the invention, which is characterized in that at the time of forming the second semiconductor layer, an upper surface of the uppermost barrier layer is caused to disappear and the uppermost barrier layer is made to have substantially the same thickness as the other barrier layers.
Further, an object of the invention is to provide a semiconductor light-emitting device in which the peak wavelengths of emitted light do not vary even if an applied current is varied.
Another object of the invention is to provide a semiconductor light-emitting device having a narrow wavelength distribution, i.e., a semiconductor light-emitting device that emits light that is close to ideal monochromatic light.
Still another object of the invention is to provide a semiconductor light-emitting device that has high light-emitting efficiency and an active layer of superlattice structure exhibiting strong emission of light.
In the semiconductor light-emitting device according to the first aspect of the invention, layers that are adjacent to the first semiconductor layer and the second semiconductor layer are barrier layers in the light-emitting layer. Therefore, the shape of a quantum well, i.e., the potential wells in the quantum well layers closest to the respective semiconductor layers are substantially the same as those of the other quantum well layers. As a result, the wavelengths of light emitted from the respective quantum well layers become substantially the same.
Further, a crystal of the barrier layer made of InY1Ga1xe2x88x92Y1N in the light-emitting layer is grown on the first semiconductor layer made of n-type GaN. Since the indium mole fraction of the barrier layer is zero or relatively smaller than that of the quantum well layer, the composition of the barrier layer is closer to that of the first semiconductor layer. Therefore, the crystal of the light-emitting layer is hard to distort.
In a method of manufacturing a semiconductor light-emitting device according to the second aspect of the invention, the uppermost barrier layer is made thicker than the other barrier layers. Therefore, even if the material on the surface of the uppermost barrier layer is caused to disappear at the time of forming the second semiconductor layer, there is no likelihood that the entire part of the uppermost barrier layer will not substantially exist. In order to allow the uppermost barrier layer to exist, the thickness of the uppermost barrier layer must, of course, be designed while taking into consideration a part of thickness to be caused to disappear at the time of forming the second semiconductor layer.
According to the third aspect of the invention, the uppermost barrier layer is designed while taking into consideration a part of thickness to be caused to disappear at the time of forming the second semiconductor layer so that the uppermost barrier layer has the same thickness as the other barrier layers after the second semiconductor layer has been formed. As a result, the respective barrier layers have substantially the same thickness in the superlattice-structured light-emitting layer, which in turn contributes to preventing wave wavelength shift caused by the quantum effect.
Further, according to a mode of embodiment of the invention, the peak wavelengths of light emitted from the semiconductor light-emitting device remain substantially unchanged even if an applied current is changed.
Still further, according to another mode of embodiment of the invention, a wavelength distribution of light emitted from the semiconductor light-emitting device becomes narrow. That is, light close to ideal monochromatic light can be emitted from the semiconductor light-emitting device.
Still further, according to still another mode of embodiment of the invention, light-emitting efficiency is high and light-emitting strength is increased in the superlattice-structured light-emitting layer.